In the initial stage of semiconductor industries, bubbling was in wide use where a carrier gas such as oxygen or the like was passed through water in a bubblier. Although this technique was advantageous in that a wide range of a moisture content could be covered, a problem on pollution could not be avoided, and thus, the technique is rarely used at present. Accordingly, an oxygen and hydrogen combustion method, i.e. a pyrogenic system, has been widespread in order to avoid the disadvantage of the bubbler.
(Disclosure of Prior Art Literature, etc.)
With regard to an improvement in thermal oxidation and a moisture generation method thereto, to which the invention is directed, the following prior art techniques are known.
(1) In Japanese Patent Laid-open No Hei 6-163517 of Ohmi, there is described a low temperature oxidation technique of lowering temperatures in a semiconductor process. In Example 1 of this application, there is set out a method wherein hydrogen is added to a gas atmosphere comprising about 99% of argon and about 1% of oxygen in an amount of from 100 ppm to 1%, from which steam is obtained at a hydrogen combustion temperature of 700° C. or below, particularly, at 450° C. or below, by the action of a stainless steel catalyst. Moreover, in Example 2 of the application, it is stated to thermally oxidize silicon in an atmosphere consisting of 990% of oxygen and 1% of steam formed by use of a catalyst at normal pressures or under pressure at an oxidation temperature of 600° C.
(2) Japanese Patent Laid-open No Hei 7-321102 (Yosikoshi) describes high temperature thermal oxidation on silicon surfaces at an oxidation temperature of 850° C. at a very low moisture concentration, i.e. 0.5 ppm of a very super low moisture content region or in a dry region, in order to avoid various problems ascribed to moisture.
(3) In Japanese Patent Laid-open No. Sho 60-107840 of Honma et al, there is described a thermal oxidation method of silicon wherein in order to reduce variations in moisture content caused by moisture in a dry oxidation environment, a very small content of moisture at a level of about several tens of ppm formed according to a conventional method is purposely added.
(4) Japanese Patent Laid-open No. 5-152282 (Ohmi I) discloses a thermal oxidation apparatus which has a hydrogen feed pipe whose inner surfaces are constituted of Ni (nickel) or a Ni-containing material in order to prevent the generation of particles from the tip of a quartz tube as set out hereinabove, and also has means for heating the hydrogen gas feed pipe. In this thermal oxidation apparatus, water is formed by bringing hydrogen into contact with Ni (or the Ni-containing material) inside the hydrogen gas feed pipe heat to 300° C. or over, and reacting the hydrogen activated species with oxygen or (an oxygen-containing gas). More particularly, water is formed according to a catalytic system involving no combustion, so that there is no possibility that the hydrogen feed pipe melts at its tip end to cause particles to be generated.
(5) Japanese Patent Laid-open No. Hei 6-115903 (Ohmi II) discloses a moisture generating method using a catalyst system which comprises the mixed gas-preparing step of mixing oxygen, hydrogen and an inert gas to prepare a first mixed gas, and the moisture-generating step wherein the first mixed gas is introduced into a reactor tube constituted of a material, which has the catalytic action and is capable of conversion of hydrogen and oxygen into radicals and the reactor tube is heated to cause the hydrogen and oxygen present in the first mixed gas to be reacted thereby causing water to be generated.
According to this method, a catalytic material, with which the reaction is able to proceed at lower temperatures, is used as the reaction tube for reaction between hydrogen and oxygen. Eventually, generation of water is enabled at low temperatures. Accordingly, where the mixed gas of hydrogen, oxygen and an inert gas is fed to a heated reaction tube, hydrogen and oxygen undergo complete reaction therebetween in the reaction tube at a temperature of 500° C. or below. Thus, a gas containing moisture can be obtained at temperatures lower than that of a combustion system.
Moreover, if a metal material alone is used for a gas contact portion after exclusion of all plastic materials therefrom and the metal surfaces are subjected passivation treatment, gases (moisture, hydrocarbons and the like) released from the surfaces become very small in amount. This permits more purified moisture to be generated in higher accuracy in a wide range of concentration (covering ppb to %). The passivation treatment is performed by thermally treating a stainless steel, which has been subjected to electrolytic polishing or electrolytic composite polishing, in an acid or weakly acidic atmosphere with an impurity concentration of several ppb or below.
(6) Japanese Patent Laid-open No. Hei 5-141871 (Ohmi III) discloses a thermal treatment apparatus which includes, as least, an opening capable of opening and closing it, through which an article to be treated is carried out and in, a furnace core tube having a gas supply port through which a gas is supplied thereinto, a heating means for heating the inside of the furnace core tube, a gas supply tube connected in communication with the gas supply port, and heating means for heating the gas supply pipe wherein at least inner surfaces of the gas supply pipe is made of Ni (or a Ni-containing material).
This thermal oxidation apparatus is provided with a hydrogen activated species-generating means for forming hydrogen activated species from a hydrogen gas or hydrogen-containing gas without involving generation of a plasma, which is located upstream of a position of an article to be treated which is placed inside the furnace core tube. A hydrogen gas or hydrogen-containing gas is introduced into the hydrogen activated species-generating means to generate activated species of hydrogen. To this end, if a silicon substrate formed with an oxide film thereon is, for example, placed in the furnace core tube as an article to be treated, the activated species of hydrogen diffuse into the oxide film and contributes to termination of dangling bond in the oxide film and at the interface of the oxide film/silicon. Thus, it can be expected to obtain a gate oxide film of high reliability.
(7) In Japanese Patent Laid-open No. Hei 5-144804 of Nakamura et al, there is set forth a technique of thermal treatment of a silicon oxide film with activated species of hydrogen formed by use of a nickel catalyst.
(8) At pages 128 to 133 of the Lecture Papers at the 45th Symposium of the Semiconductor Integrated Circuit Techniques promoted by the Committee of Electronic Materials of the Association of Electrochemistry, there is reported a silicon oxidation process in a strongly reductive atmosphere mainly comprising hydrogen radicals produced by use of a catalyst for application to a tunnel oxide film of flash memories and hydrogen from moisture.
(9) In Japanese Patent Laid-open No. Hei 6-120206 of Ohmi, there is described a sintering technique using hydrogen activated species which are produced by means of a nickel catalyst for an insulating film insulating and isolating a selective epitaxial growth region therewith.
(10) In Japanese Patent Laid-open No. Sho 59-132136 of Kobayashi et al, there is set out a process of oxidizing and reducing silicon and a refractory metal in an oxidative and reductive mixed atmosphere of moisture and hydrogen generated by an ordinary method